Passive low frequency RFID readers and tags use operating principles that are well-know to those of ordinary skill in the art, and that are described in extensive detail in several seminal inventions, including U.S. Pat. No. 1,744,036 to Brard, U.S. Pat. No. 3,299,424 to Vinding, U.S. Pat. No. 3,713,146 to Cardullo, and U.S. Pat. No. 5,053,774 to Schuermann, and in textbooks such as Finkenzeller, “RFID Handbook” (1999).
International Standards Organization (ISO) Standard 11785, “Radio frequency identification of animals—Technical Concept” (1996) defines the technical principles for communications between reader devices and two types of electronic passive identification transponders. Both transponder types contain identification information stored in binary form, which is conveyed to the reader when a transponder is suitably activated by the reader. ISO 11785 relates to radio frequency identification (RFID) systems, comprising low frequency reader devices and passive, inductively powered identification tags (i.e., “ID tags”). In RFID systems of this type, the reader device generates a tag activation signal, and receives identification data signals from the ID tag. Such a reader device can use separate transmit and receive antenna elements to perform these functions. Additional technical details are provided in ISO Standard 11785, which is incorporated into this disclosure by reference in its entirety.
Readers in which a single antenna performs both transmit and receive functions are very cost effective and efficient, and comprise the most common design implementation in low-frequency RFID readers. However, when a single antenna is used for both transmit and receive purposes, the antenna's design characteristics must be inevitably compromised, and the antenna performs less efficiently than when it can be designed optimally for a single purpose.
A conventional RFID reader including a single resonant antenna is shown in FIG. 1. The RFID reader [100] includes electronic circuitry, which generates an activation signal (usually a single frequency unmodulated signal) using a signal source [101] and an amplifier [102] to drive a resonant antenna circuit [103]. This activation signal is manifested as a time-varying electromagnetic field, which couples with the ID tag [105] by means of the electromagnetic field's magnetic field component [104a]. The ID tag [105] converts this magnetic field into an electrical voltage and current, and uses this electrical power to activate its internal electronic circuitry. Using any of several possible modulation schemes, the ID tag conveys binary encoded information stored within it back to the reader via magnetic field [104b], where the detector and utilization circuit [106] converts this binary code into alphanumeric format tag data [107] in accordance with some prescribed application.
FIG. 2 shows the composition of FIG. 1's resonant antenna circuit [103] in schematic detail within the dashed line [212]. The resonant antenna circuit includes at least one capacitor C [213] connected to at least one inductor L [214], where the values of C and L are selected such that the circuit resonates at the signal source frequency [210] as amplified by amplifier [211]. Inductor L is also constructed in such a manner that it creates a magnetic field [215] within its immediate vicinity.
ISO Standard 11785 defines two types of transponder technologies, which are designated “full-duplex” (“FDX-B”) and “half-duplex” (“HDX”). In the described manners that follow, for FDX-B and HDX transponders, respectively, activation energy is transferred to the transponder from the reader, and identification code information is transferred to the reader from the transponder through the mutual coupling of a magnetic field.
The FDX-B transponder communicates to the reader by amplitude modulating the activation signal it receives with a binary pattern representative of the data stored within the tag. Amplitude modulation imposes variations on the activation signal's magnitude, and the reader is equipped with sensing circuitry capable of detecting these magnitude variations. The term “full-duplex”is indicative of the fact that the FDX-B transponder sends its identification code information during the time when it is receiving the activation signal from the reader.
An HDX transponder typically contains its own micro-power radio frequency transmitter, which is powered with energy received from the reader's activation signal and stored internally in a capacitor component. Once the activation signal ceases, the HDX transponder emits a very low strength radio signal, comprising a frequency shift keyed (“FSK”) modulation scheme. Specifically, the binary identification code information contained in the HDX tag is serially output such that the occurrence of a binary “1” results in the HDX tag's radio signal being 124.2 KHz and a binary “0” results in the tag's radio signal being 134.2 KHz. The reader detects this FSK signal and derives from it the HDX transponder's identification code. The term “half-duplex” is indicative that the reader and the HDX transponder exchange the activation signal and the identification code signal in alternating time intervals.
An ISO compliant reader has the capability to activate and detect both FDX-B and HDX type transponders when present. To accomplish this, the ISO compliant reader transmits an activation signal, consisting of a 134.2 kilohertz (KHz) sinusoid, which is switched ON and OFF in a prescribed pattern in accordance with ISO 11785. During the interval in which the 134.2 KHz signal is ON, the FDX-B transponder is activated and it transmits its identification code signal cyclically for as long as the activation signal is present. During this ON interval also, an HDX transponder charges its internal capacitor. Subsequently, during the interval in which the 134.2 KHz activation signal is OFF, the FDX-B transponder remains dormant, and the HDX transponder transmits its identification code sequence.
FIGS. 3(a) through 3(c) illustrate the frequency spectrum characteristics of an RFID system conforming with ISO 11785. FIG. 3(a) shows the spectra for the HDX tag, where the activation signal [310] appears at 134.2 KHz, and where the HDX transponder frequencies appear at 124.2 KHz [311] and 134.2 KHz [312]. Since the activation signal and the HDX transponder signals are time interleaved, the 134.2 KHz activation signal [310] and the 134.2 KHz transponder signal [312] typically do not occur simultaneously. Thus, the reader's receive circuitry is able to detect the transponder frequency without interference from its own activation signal.
FIG. 3(b) shows the spectra for the FDX-B tag, where the activation signal [320] appears at 134.2 KHz, and where the FDX-B transponder's amplitude modulation appears as sidebands [323] close to the 134.2 KHz carrier. As is well known to those of ordinary skill in the art, amplitude modulation sidebands appear symmetrically around the modulated carrier signal, and for FDX-B specifically, these sidebands appear at +2.097 KHz and +4.194 KHz. Because the activation signal [320] and the data signal [323] are distinct frequencies, they can occur simultaneously, and the reader is able to separate the two signals, thus recovering the tag data contained in these sideband frequencies.
In FIG. 3(c), the frequency spectral characteristics from FIG. 3(a) and FIG. 3(b) are shown together, along with curve [335], which characterizes the frequency response of the reader's resonant antenna circuit [212] of FIG. 2. For the resonant antenna circuit to perform well as both transmitter and receiver, and for both HDX and FDX-B tags, the antenna design is typically a compromise. The resulting resonant antenna [212], [335] functions adequately as both transmitter and receiver for HDX and for FDX-B, but in having a fixed Q-factor and a fixed resonant frequency that works for all its functions, it is not optimized for each individual function. For example, in order to be an efficient 134.2 KHz activation signal transmitter, the resonant antenna circuit is ideally characterized by a very high quality factor, or “high-Q”, as it is known to those skilled in the art. The “Q-factor” of an inductor used in a resonant circuit (such as the antenna is) describes “sharpness” or “selectivity” of the inductor. Mathematically, the resonant antenna's Q is calculated according to the formula:
  Q  =                    2        ⁢                                  ⁢        π        ⁢                                  ⁢        fL            R        =                            (                      L            /            C                    )                          1          /          2                    R      
where f is the resonant frequency, L is the inductor's inductance value, C is the resonant capacitance, and R is the inductor's resistance. Furthermore, the bandwidth of a resonant antenna circuit using such an inductor is:BW=f/Q 
Thus, a resonant antenna circuit has a very high-Q when its resistance is very low, but this very high-Q implies a very narrow bandwidth. As shown in FIG. 3(c), a resonant antenna circuit with a very high-Q exhibits a narrow bandwidth as depicted by the curve [336]. A very high-Q resonant antenna circuit is very effective and efficient for transmitting a single frequency activation signal [330], but has insufficient bandwidth to capture the HDX [331], [332] and the FDX-B [333] transponder spectra. Thus, a very high-Q antenna typically does not work satisfactorily as both a transmitter and a receiver antenna.
If the resonant antenna circuit's Q is decreased such that it exhibits the bandwidth depicted by curve [334], the antenna is less efficient in transmitting the activation signal, but provides sufficient bandwidth to capture the FDX-B transponder spectra [333]. However, the lower HDX data frequency at 124.2 KHz [331] lies outside the antenna's response curve, and typically will not be effectively captured.
If the resonant antenna's Q-factor is decreased even further such that it exhibits the bandwidth depicted by curve [335] in FIG. 3(c), this “low-Q” wide-bandwidth characteristic will adequately capture both the HDX [331], [332] and FDX-B [333] transponder spectra, but the antenna does not transmit the activation signal efficiently. Furthermore, this wider bandwidth makes the antenna more susceptible to interference signals from other nearby electromagnetic radiating sources, and this can be especially detrimental to FDX-B performance.
From the curves [334], [335], and [336] in FIG. 3(c), it is apparent that as the resonant antenna's Q-factor is increased to improve transmit efficiency and interference rejection, the antenna becomes less suitable for transponder signal reception. However, if the resonant antenna's Q-factor is dynamically altered, the antenna can function more efficiently as transmitter and receiver for both HDX and FDX-B type transponders.
An improvement to the resonant antenna circuit thus far described is disclosed in U.S. Pat. No. 7,528,725 to Stewart, the fundamental principles of which are reproduced herein in FIGS. 4(a) through 4(d). The '725 Stewart patent proposes the use of a resonant antenna having an intermediate Q-factor, such as that depicted in FIG. 3(c) by curve [334] and in FIG. 4(b) by curves [414] and [415]. During the transponder activation period when FDX-B transponder data is present, the antenna's resonant frequency is set to 134.2 KHz [410], and it's Q-factor provides sufficient bandwidth to capture the FDX-B transponder spectra [413]. When the activation signal ceases, and the HDX transponder signal becomes present, the antenna's resonant frequency is lowered to nominally 129.2 KHz as shown by curve [415], thus allowing the HDX transponder spectra [411], [412] to be captured. The disclosure of U.S. Pat. No. 7,528,725 to Stewart is hereby incorporated by reference in its entirety.
FIG. 4(a) illustrates an electrical circuit that can accomplish the resonant frequency shifting described in the '725 Stewart patent using the activation signal's on/off state. When the activation signal [406] is present, it drives the resonant circuit comprising capacitor C [404] and inductor L [405]. The activation signal bypasses inductor LT [403] through diodes D1 [401] and D2 [402], and so the resonant frequency of the antenna is determined by the values of C [404] and L [405], which are selected to resonate at 134.2 KHz. When the activation signal is absent, diodes D1 [401] and D2 [402] become high impedances, and thus inductor LT becomes part of the antenna circuit. The resonant frequency of the antenna is now determined by the values of C [404], L [405], and LT [403], which are selected to resonate at 129.2 KHz. Thus, as the activation signal [406] is switched on and off, the antenna's resonant frequency dynamically shifts between 134.2 KHz [414] and 129.2 KHz [415].
The '725 Stewart patent also discloses the possible change in Q-factor during the resonant frequency shifting process, as shown in FIGS. 4(c) and 4(d). If inductor LT [425] has a non-zero resistance value RT [426], or if a discrete resistive component is inserted into the circuit to create resistance RT [426], the resulting effect is depicted in FIG. 4(d). During the activation signal on interval, the antenna has a resonant frequency of 134.2 KHz and a Q-factor as shown by curve [434]. When the activation signal is turned off, inductance LT [425] and resistance RT [426] are inserted into the antenna's resonant circuit, thus altering the antenna's characteristic to that shown by curve [435] in FIG. 4(d).